1. Field of the Invention
This invention relates to electromagnetic contactors.
2. Description of the Prior Art
Conventional alternating current (AC) operating magnets are designed usually to operate over a range of 85% to 110% of the rated power system voltage. Because of the alternating current, the operating magnet field goes through zero every 1/2 cycle. Shading coils are required to provide some degree of field during this period. They act as short-circuited secondary windings on a transformer and also prevent chatter and reduce noise in addition to providing a holding force. AC magnets typically drop out in a range of 60 to 70% of rated operating voltage.
In the open condition, an AC magnet has a relatively high inrush current and provides a high magnetic pull-in force. The AC impedance is very low and the current is limited mainly by the direct current (DC) resistance of the operating coil. As the magnet air gap decreases, the AC impedance increases. In the magnet-sealed position, the AC impedance is relatively high and the current is reduced to a low value that will not overheat the coil. This characteristic functions as an automatic "regulator" which provides a large pull for closing in and a sufficient smaller pull to hold the magnet closed when sealed. AC magnets are generally much smaller than continuously rated DC magnets which produce the same pull-in or open-condition magnetic forces.
Many rural irrigation pumping applications are located at the end of limited power systems where voltage drops are frequent and severe. The starting of a large pump motor may cause the voltage to drop to 50% or more. This voltage dip may, in turn, cause other control equipment to shut down or malfunction. Unless an excessively large AC magnet with respect to its mechanical load tending to drop out the magnet armature is used, it is customary to use a DC magnet to secure low dropout operation.